To butterfly the flank steak, place the steak at the edge of a cutting board with a short side toward you and the edge of the narrower long side on your right (reverse if you are left-handed). Using a long, thin-bladed knife, cut the meat almost in half through the narrower side, from right to left (or vice versa). Open it as you would a book. With a mallet, pound the steak lightly to an even thickness. Ideally, you want a round square of meat that is 12-15 inches (30-38 cm.).

Preheat the oven to 350 degrees F (180 degrees C).

Lightly season the cut side of the meat with salt and pepper. Lay the prosciutto slices in an even layer running across the grain from top to bottom. In a bowl, combine the garlic, oregano, sage, olives, cheese, bread crumbs, vinegar, and 2 tablespoons of the oil. Stir to blend evenly. Spread the mixture evenly over the prosciutto, leaving 3 inches (7.5 cm.) of one side of the meat uncovered. Roll up the meat around the filling, working toward the uncovered side, to make a compact roll. The roll should be about 4 inches (10 cm.) in diameter. Tie it at regular intervals with kitchen string.

Choose a roasting pan large enough to accommodate the rolled meat. Place the pan on the stove top over medium-high heat, and warm the remaining 1 tablespoon oil. Add the roll and cook, turning as needed, until browned well on all sides, 12-15 minutes total. Place in the oven and cook until an instant-read thermometer inserted into the center of the roll registers 130-135 degrees F (54-57 degrees C) for medium-rare, 25-30 minutes. Transfer to a cutting board, tent loosely with aluminum foil, and let rest for 15-30 minutes.